This grant proposal is submitted under PA-18-872 Research to advance vaccine safety. The impetus for this R21 grant proposal is a recent report from this lab in the Journal of Child Neurology 2019. The title of the article: Severe herpes zoster following varicella vaccination of immunocompetent young children. The hypothesis for this proposal is that VZV ORF0 is a major determinant of attenuation in the live varicella vaccine vOka; further, a variant of the current vaccine product that contains the wild-type ORF0 allele rather than the attenuated ORF0 allele is more likely to reactivate as severe herpes zoster. My lab and a Canadian lab made a critical discovery about VZV attenuation after we sequenced the entire genome of the well-known Ellen lab strain. We had previously shown, using the Arvin lab?s SCID mouse model, that the Ellen strain was even more attenuated than the Oka vaccine strain. When we compared the complete sequence of Ellen and vOka, we made a completely unpredictable discovery. The two strains had common SNPs in IE62 gene, as expected. What was not expected was the discovery that the Ellen lab strain (a wild type strain isolated in Georgia in the 1960s) had a vaccine-type ORF0 gene. Unbeknownst to VZV researchers, therefore, the Ellen strain had acquired polymorphisms in both IE62 (ORF62) and ORF0 that were extremely similar to the vaccine strain. The goal of the R21 grant is to initiate studies to define ORF0 as an important determinant of attenuation in the VZV vaccine. Essentially, therefore, this grant proposes that the virologists who first made the live varicella vaccine in the 1970?s omitted a step in the late design of the vaccine, which allowed a more virulent variant to be included in the final vaccine product. The research plan includes two specific aims and also includes two consultants. The importance of this research has vaccine safety relevance. There have been thousands of reports of adverse event after varicella vaccination, including death from disseminated vaccine virus infection. Since the original stocks of vaccine virus were never cloned by the Takahashi lab in Osaka, the current commercial vaccine product contains a variant with wild-type ORF0 allele. We suggest that the vaccine subspecies or variant with wild-type ORF0 may be causing many of the severe side effects following varicella vaccination.